A fiber optic has been used as a sensing element, referred to as an “intrinsic sensors” or as a means for transmitting signals from a sensor, referred to as an “extrinsic sensor,” to a signal processing unit. Intrinsic optical fiber sensors can be used to measure strain, temperature, pressure, flow, acoustic information and other parameter quantities through the variations in one or more of the intensity, phase, polarization, wavelength or time delay of the light signal transmitted in the optical fiber. For example, to measure a temperature, an optical fiber may be modified to have evanescent loss that varies with temperature changes. Further, the Rayleigh scattering, Raman scattering or the Brillouin scattering in the optical fibers may be analyzed to detect temperature variations.
Optical fiber sensors have been used for downhole applications in oil/gas wells for distributed temperature sensing (DTS) as well as pressure, flow, and acoustic sensing applications, as disclosed, for example, in U.S. Pat. No. 9,874,084. The fiber optic sensor is well suited for this environment because of its relative resilience to high temperatures and/or high pressure environments that are challenging for other type of sensors, e.g., semiconductor-based temperature sensors. For example, in a borehole, the temperature may be as high as 130° C. and the hydrostatic pressure may be as high as 1000 bar, which will damage semiconductor-based sensors.
An optical fiber sensor may be inserted into a wellbore, e.g., by running along a sidewall of the tubular borehole to measure the temperature, pressure, flow, or other parameters within the wellbore downhole. See, for example, U.S. Pat. No. 7,773,841. An optical fiber may also be pumped into a dedicated monitoring borehole, other than a production wellbore, which is dedicated to measure the earth formation parameters adjacent to the production borehole.
US 2012/0039359 relates to an integrated high temperature downhole DTS (distributed thermal sensing) sensing cable which includes sensing fibers connected to a turnaround section via fusion splices. An inner sleeve, such as a metal tube, is mounted to the sensing fibers and positioned along the length of the sensing fibers ending slightly away from the splice junctions. An elongated outer casing is attached to the inner sleeve toward the terminating end of the sensing fibers. The casing includes an outer armor tube that is affixed to the inner sleeve toward the remote end thereof. The terminating end of the armor tube defines an opening that is positioned away from the sensing fibers. The casing also includes a sealing cap that is affixed to the armor tube, welded to the armor tube.
U.S. Pat. No. 7,269,320 (AFL) provides a fiber optic cable for use in temperature measurement in the control line of an oil well having a pair of optical fibers connected by a miniature bend at one end. The miniature bend is surrounded by a protective casing. The fibers and the protective casing are contained within a tube that may be made from a metal. The tube includes a seal at its terminating end. A jacket may surround the seal and the tube.
The already cited U.S. Pat. No. 9,874,084 discloses a coiled tubing systems, i.e. a relatively small diameter continuous tubing string, to deploy more instrumentation into the wellbore, particularly pressure and temperature sensors. An end cap assembly is welded in place at the bottom hole end of the coiled tubing string containing, for example, two metal conduits containing fiber optic sensors. The sealing is accomplished by providing a flat weldable surface to which the end of the coiled tubing string is welded.